Fabrication of thermoelectric elements

ABSTRACT

An isostatic hot compressing process step is employed to exert a pressure of from 5,000 p.s.i. to 50,000 p.s.i. on the exterior surfaces only of a hollow cylindrical thermoelectric element having thermoelectric material disposed between exterior and interior cylindrical shell members to plastically deform the exterior surfaces of the element and reducing the annular crosssectional area of the element from 1 percent to 15 percent to provide at least an intimate physical contact between the body of thermoelectric material contained therein and the inner and outer members of the element. The inner member of the element is in compression after the hot isostatic compression is removed. Isostatic cold compressing may be applied to the element prior to the isostatic hot compressing process step.

' UnitedStates Patent Continuation-impart of application Ser. No. 7593,528, Nov. 10, 1966, now abandoned.

FABRICATION 0F THERMOELECTRIC ELEMENTS 13 Claims, 6 Drawing Figs.

1m. 01 B01 j 17/00, 11011 15/00, 110111 49/00 Field of Search PrimaryExaminerJohn F. Campbell Assistant Examiner-D. M. Heist Attorneys-F.Shapoe and C. L. Menzemer ABSTRACT: An isostatic hot compressing processstep is employed to exert a pressure of from 5,000 p.s.i. to 50,000p.s.i. on the exterior surfaces only of a hollow cylindricalthermoelectric element having thermoelectric'material disposed betweenexterior and interior cylindrical shell members to plastically deformthe exterior surfaces of the element and reducing the annularcross-sectional area of the element from 1 percent to 15 percent toprovide at least an intimate physical contact between the body ofthermoelectric material contained therein and the inner and outermembers of the element. The inner member of the element is incompression after the hot isostatic compression is removed. Isostaticcold compressing may be applied to the element prior to the isostatichot compressing process step. 1

PATENTEB M1831 I971 sum 1 or 2 FIG.|.

FIG.2.

FIGS.

FIG.4.

NORMALIZED EFFICIENCY NORMALI ZED THERMAL CONDUCTANCE ENTEU AUGS] IBH3501.8 7

sum 2 [IF 2 O-TYPICAL POINTS FROM (6) ELEMENTS FABRICATED BY PRIQR ARTPROCESS (EXAMPLE I) x -:l "I

g ELEMENTS FABRICATED ,5 EXAMPLE I AJ4 BY NEW PROCESS A V J5 EXAMPLE m gI I 6 EXAMPLE m I00 200 300 400 soo e00 "/00 B00 9'00 OPERATING TIME-HOURS FIG.6.

O- TYPICAL POINTS FROM (6) ELEMENTS FABRICATED OPERATING TlME HOURSFABRICATION OF THERMOELECTRIC ELEMENTS This application is acontinuation-in-part of our application Ser. No. 593,528, filed Nov. 10,1966, now abandoned, the assignee of which is the same as that of thepresent application.

The present invention relates to a process for preparing thermoelectricelements.

A prior process employed to fabricate hollow cylindrical thermoelectricelements which comprise an outer cylinder member and a smaller innercylindrical member disposed therein with the thermoelectric materialproper being placed in the space between them, utilizes apparatus andtechniques exerting a high pressure upon both the outer surface of theouter cylindrical member and the inner surface of the inner cylindricalmember of the elements. The application of the pressure at a hightemperature upon both outer and inner cylindrical surfaces reduces thediameter of the outer cylindrical member and increases the diameter ofthe inner cylindrical member. It should be noted that the innercylindrical member is stretched or elongated in the process. Thisprocess eliminates radial assembly gaps between the components of theelement, densifies the thermoelectric material and establishes a goodthermal contact at each radial interface.

However, when an element processed in this manner is operated at anelevated temperature, the inner cylindrical member has reducedefficiency. This has been found to result from the fact that theinnermost member relaxes inwardly, thereby reducing its contact pressureupon the adjoining components of the element. The resulting reduction incontact pressure increases the thermal contact resistances within theelement, thereby causing a reduction in the efficiency of the element.

An object of this invention is to provide a process for producing hollowcylindrical thermoelectric elements having the innermost portions incompression whereby the innermost cylindrical portions do not undergorelaxation when the elements are operated at a high temperature.

Another object of this invention is to provide a process formanufacturing a hollow cylindrical thermoelectric element by applyingisostatic pressure only to the outer surface whereby the element iscollapsed onto a heat source within the hollow member, thereby assuringa good thermal conductivity relationship between the cylindrical elementand heat source, therebypreventing inward relaxation of the thin wallmember when the element is operated at a high temperature.

A further object of this invention is to provide a process formanufacturing a hollow cylindrical thermoelectric element embodying athick wall inner cylindrical member .to develop a compressive stress onthe innermost portions adjacent the thick wall whereby when the elementoperates at a high temperature the thick wall inner cylindrical membermaintains good thermal contact with the adjoining components of theelement.

Other objects of this invention will, in part, be obvious and will, inpart, appear hereinafter.

For a better understanding of the nature and objects of this invention,reference should be had to the following detailed description anddrawings, in which:

FIGS. l and 2 are perspective views, partly in cross section, ofthermoelectric elements made in accordance with the teachings of thisinvention;

FIGS. 3 and 4 are views, partly in cross section, of ther moelectricelements made in accordance with the teachings of this invention;

FIG. 5 is a graph comparing the normalized overall efficiency ofthermoelectric elements made by a prior art process and compared withthermoelectric elements made in accordance with the teachings of thisinvention; and

FIG. 6 is a graph comparing the normalized thermal conductance ofthermoelectric elements made by a prior art process and compared withthermoelectric elements made in accordance with the teachings of thisinvention.

In accordance with the'present invention and in attainment of theforegoing objects there is provided a process for producing an integralthermoelectric element, the steps comprising disposing a thermoelectricmaterial, or a plurality of layers of thermoelectric material, within ahollow compartment defined by the outer wall of an inner metal memberand the inner wall of a larger diameter outer metal member, the metalmembers being concentric with each other, sealing the ends of thecompartment to retain the thermoelectric material therein and to preventsubstantial pressure developing in the hollow, preferably evacuating thecompartment, and hot pressing the entire assembly at a temperature fromabout 250 C. to slightly below the melting temperature of the lowestmelting temperature component in the assembly by the application of anisostatic pressure of from 5,000 p.s.i. to 50,000 p.s.i. or higher to aselected surface area of only the outer metal member to effect areduction of from 1 percent to 15 percent in the cross-sectional area ofthe compartment to provide at least an intimate physicalcontact, and insome instances a metallurgical bond between the thermoelectric materialand the walls of the metal members and increasing the density of thethermoelectric material by 4 percent to ID percent. The process resultsin a compressive stress being imparted to the innermost layers after thepressure is released.

In one preferred embodiment of the invention, a plurality of compressedbodies of powdered thermoelectric material are assembled within thespace between an inner and outer cylindrical metal member. Electricalinsulation is interposed between portions of the bodies. Bridgingelectrical contacts are disposed between certain portions of the bodiesof thermoelectric material with the insulation being disposed betweenthe cylindrical members and the bridging members, to provide acloselypacked assembly. The inner cylindrical metal member is hollow andhas a thin wall and this hollow is completely filled with a solid heatsource such, for example, as a radioisotope, or a nuclear reactor fuel,for example U0 and is sealed to prevent fluid pressure developingtherein during subsequent processing.

The entire assembly is hot pressed at an isostatic pressure of fromabout 5,000 p.s.i. to 50,000 p.s.i. at a temperature of from about 250degree C. to the melting temperature of the lowest melting temperaturecomponent of the assembly until the space between the inner and outercylindrical members has a reduction in cross-sectional area of fromabout 1 percent to 15 percent to provide a metallurgical bond betweenthe thermoelectric bodies, the metal members and the bridging electricalcontacts and to increase the density of the thermoelectric material byabout 1 percent to 10 percent. The thin wall of the inner cylindricalmember will be collapsed upon the surface of the solid heat source sothat the two are in an intimate physical contact and a good thermalconductivity relationship with each other.

In an alternate and preferred embodiment of the invention, thecomponents are the same as in the first preferred embodiment except thatthe inner cylindrical member is a thick walled hollow tube. Prior toprocessing, the bore of the thick wall hollow tube is hermeticallysealed to exclude autoclave gas. One method of hermetically sealing thetube is to fill the tube with a closely fitting solid metal bar which isthen seal welded to the ends of the inner tube. Another suitable methodof hermetically sealing the bore of the tube is to insert an end pluginto each end of the tube and seal each plug therein. The tube bore maybe evacuated, filled with air, or filled with an inert gas.Additionally, various solid fillers such, for example, as solidcylindrical punchings of metal, metal bars or metal rods may be stackedor otherwise disposed within the bore of the tube prior to inserting theend plugs and welding them to the tube. The solid filler must physicallysupport the inner wall of the tube bore and be nonreactive with thematerial of the cylindrical member during processing of the device.

In each instance of the alternate and preferred embodiment of thisinvention, the assembled components are hot pressed by applying anisostatic pressure to the exterior surface of the assembled componentsonly thereby reducing the diameter of each cylindrical component layer,and radially compressing the inner cylindrical member rather thanstretching it as in prior art devices.

The thermoelectric materials employed herein may comprise metallic andnonmetallic substances such, for example, as solid or powdered metals,ceramics or semiconductors or mixtures of two or more. Thethermoelectric members may be preliminarily prepared by compressingpowdered thermoelectric materials in a suitable die to a density ofabout 85 percent and higher, or by casting in a manner known to thoseskilled in the art. The insulating materials employed as partitionmembers and the like between thermoelectric bodies may comprise any goodinorganic electrical insulators such, for example, as silica, mica,alumina, boron nitride, beryllium oxide and inorganic silicates such asboron silicate and lime glasses and those materials comprising thereaction product of mica lead borate glass sold under the trade name ofMycalex and materials comprising magnesium silicates sold under thetrade name Lavite. The cylindrical metal members may comprise a goodelectrically and thermally conductive material, such as silver,aluminum, nickel, stainless steel, pure iron and copper or base alloysthereof.

In the preferred embodiments of the invention, there is provided anintegral, elongated, thermoelectric element comprising a series ofwasher-shaped P- and N-type thermoelectric bodies electrically joined inseries within a compartment provided between an outer cylindrical metalmember and an inner cylindrical metal member electrically insulatedtherefrom but in good thermal conducting relation therewith. Thethermoelectric element includes a plurality of inner and outer bridgingmetal ring members on an insulated cylindrical metal member disposedbetween successive thermoelectric washers. The bridging ring members maybe electrically insulated from the cylindrical members by disposing arelatively thin insulating member therebetween either preparedseparately as when employing a material such a'boron nitride or mica, orproduced in situ by plasma jet spraying an insulating material, such asalumina, on the walls of the cylindrical members. When employing aseries of adjacent bridging ring members, they will be electricallyinsulated from each other. The bridging ring members may comprise anygood electrically conductive metal such, for example, as nickel, copper,aluminum, iron or base alloys thereof.

A plurality of compressed washers of powdered thermoelectric materialare disposed with one inner diameter face on the inner bridging ringmembers and an outer bridging metal ring member disposed on the outerperiphery of the compressed washer of thermoelectric material so thatthe thermoelectric washer members are electrically connected at eachperipheral face or diameter thereof by either the inner or outerconcentric bridging ring members while successive washers areelectrically insulated from each other at the other diameter orperipheral face. The insulating materials employed between thethermoelectric washers may comprise any of the electrical insulatorsdescribed previously with respect to the partition members of the lesscomplicated configurations.

The number of thermoelectric washer members employed will determine thenumber of metal bridging ring members needed to provide electricalcontact therebetween. It is preferred that each pair of thermoelectricwashers electrically contacting each other consist of a P-typethermoelectric material and an N-type thermoelectric material. Thecomponents of the assembly are as closely packed as is possible so thatthe total free or gap space in the assembly is not above about 1 percentof the diameter of the outer cylindrical member.

The resulting elongated thermoelectric member after being fullyprocessed by isostatic compressing has electrical leads attached to theend thermoelectric washers so that they may be connected in anelectrical circuit.

In certain embodiments of the invention a complete thermoelectric deviceis not produced in a single operation. Thus,

a single P or N-type thermoelectric body is bonded metallurgically tothe walls of two concentrically disposed cylindrical metal members. Inthis case the resulting isostatically processed member having its innerwalls under compressive stress may be severed into a plurality ofcylindrical units of any desired length which may be further machined toshape or size, or the member may be severed into relatively smallindividual thermoelectric pellets of desired shape. The cylindricalunits or pellets may be joined to other thermoelectric pellets ofopposite type to produce a stack of composite thermoelectric elementsand assemblies which may be electrically connected, and suitablyinsulated both electrically and thermally, into thermoelectric powergenerators or cooling devices.

It is particularly desirable in all embodiments of the invention thatthe interior of the unit being deformed or hot pressed be evacuatedprior to deformation to remove all reactive gases.

The terms isostatic pressure," isostatic deformation, isostatic coldpressing or isostatic hot pressing as used herein refer to a methodinvolving the application of high enough pressures for reducing thecross-sectional area of a member to a sufficient degree to plasticallydeform and urge adjacent components into an intimate physical contactwith each other, and even bond a component to the mating face of anotheradjacent contacting component, the pressures being uniformly applied orinduced to all the exterior surfaces by using gases and/or liquids as acompressing medium. Although the preferred pressure range is from 5,000to 50,000 psi, it should be appreciated that higher and, in someinstances, slightly lower pressures may be employed depending upon thematerials involved in the assembly, the temperature, time of applicationof temperature and pressure, and other factors. lt should be understoodalso, that an intimate physical contact, and when required, goodmetallurgical bonds between mating faces should be obtained, and wheninitial close tolerances between components are present, the amount ofdeformation required to provide the intimate physical contact and goodbonds may be 1 percent or even less.

The temperatures employed in the process are selected by reference tothe materials and their properties. In assemblies where some of thecomponents may have large differences in thermal expansivity, in orderto minimize joint stresses on cooling the joining temperatures should beselected at the lowest possible temperature which will effect asatisfactory bond of all component. Generally, the temperature chosen bythe above consideration will be best for bonding any assembly whetherthere is expansivity mismatchbetween components or not. Otherconsiderations are modulus of elasticity of the various materials andquality of the bonds in order to achieve the best bonded assembly withthe minimum of internal stresses.

The period of time of application of temperature and pressure isselected by the consideration of allowing all parts of the assembly toachieve thermal equilibrium, and that of allowing solid-state diffusionto occur to effect the metallurgical bonds between adjacent contactingcomponents of the assembly. If necessary, this latter requirement isreadily determined by experiment in each case. Experimental data hasindicated that many metallurgical bonds can be formed in as little as 15minutes of heating, but periods of 2 hours appear to be more reliable.Bonding may be speeded up, or accomplished at lower temperatures orpressure, if desired, by adding joint bond promoters of various kinds,such as rapidly diffusing elements as is well known in the art.

By the isostatic process disclosed herein, all the bonds between all thecomponents of the thermoelectric element may be formed in a singleoperation. That is, the complete assemblies are disposed in a suitablepressure vessel, such as an autoclave containing a heating coil and thedesired temperatures, pressures and times for the particular assemblyare imposed. After removal from the pressure vessel all the necessarybonds have been provided so that the only processing necessarythereafter is of a mechanical nature, suchas cleaning and applyingleads, so that the completed unit may be integrated in some type of anelectrical circuit.

Referring to FIG. 1, there is shown an isostatically hot pressedthermoelectric element 10, after one end of the element is removed as bymachining, consisting of an inner thick wall hollow cylindricalmetalcontact member 12 and an outer cylindrical concentric metal contactmember 14 with a body 16 of compressed powdered thermoelectric materialdisposed therebetween and metallurgically bonded to the walls of themetal members 12 and 14. Surprisingly, good bonding is effected betweenthe metal walls 12 and 14 with the body 16. The thermoelectric materialbody 16 may consist of any one of the P orN-type thermoelectricmaterials or two or more suitable layers in any desired arrangement orsequence.

The metals used in forming the members 12 and 14 are selected on thebasis of their compatibility with the thermoelectric material, desiredelectrical and thermal characteristics and resistance to the corrosiveatmospheres for a given application.

The wall thickness of tubular member 12 is sufficient so that the tubewill not collapse when subjected to the autoclave temperature andpressure. The entire assembly is evacuated, the materiall4 sealed offwith a cap and the ends of member 12 are capped to prevent anyappreciable pressure developing therein. When subject to the pressure ofthe autoclave of up to 50,000 p.s.i. or more at the desired temperature,the inner tube 12, as well as the adjacent layers of material 14, arecompressed rather than stretched as in prior art devices. in subsequentservice upon being heated to a high temperature, the member 12 expandspartly due to the retained compressive stress, whereas prior art devicescontracted. The ability of the member 12 to expand at the hightemperature enables it to remain in a good thermal conductivityrelationship with the thermoelectric component 16 of the element therebypreventing degradation of efficiency of element 10 due to loss ofcontact pressure between adjacent components.

When employing the isostatically hot pressed thermoelectric element 10or a section thereof in an operational device, it is often desirable toconnect two or more of either P or N-type or alternate P-N-type elementsin a particular type of arrange ment and circuitry.

If it comprises the hot junction of a refrigerating device, the innerhollow contact member12 not only serves to carry electrical current, butenables a cooling. fluid such as water or air to be conveyed todissipate heat. If the element 10 is employed as part of an electricalgenerator, hot gases, liquid or other heat source may be disposed in orpassed through the hollow contact member 12. The outer contact member 14may coola space or it may dissipate heat to a cold sink in either ofthese cases. The functions of the outer contact member 14 and the innercontact member 12 can be reversed.

With reference to FIG. 2 there is shown a thermoelectric element 20which is another desirable embodiment of this invention.

The element 20 comprises two concentric cylindrical metal members, anouter member 22 and an inner member 24. A body 26 of thermoelectricmaterial is disposed in the compartment defined by the outer wall of theinner member 24 and the inner wall of the outer member 22. A solid heatsource 28 such, for example, as a radioisotope or a nuclear reactor fuelsuch as U0 disposed in a refractory, is disposed within and closelyfills the hollow inner member 24. The assembly is evacuated and plugsseal up and ends 22 and 24 so that no hydrostatic pressure entersinteriorly. I

The inner member 24 has a thin wall capable of being collapsed upon thesolid heat source when subjected to the temperature and pressure of theautoclave during the isostatic compression of the element 20. Further,the wall 24 is put into compression by the pressure transmitted fromwall 22 through the consolidation of material 26. A dense unitaryelement 20 is produced wherein the heat source'28 is in intimate contacttact with material 26 and so on. The heat source 28 is also incompression after the isostatic compression process step so that goodheat flow occurs.

Referring to FIG. 3, there is shown a thermoelectric device 30comprising an isostatically deformed complete thermoelectric element 40embodying the teachings of this invention. The element 40 comprises athick wall inner cylindrical metal member 42 and a concentric outercylindrical metal member 44.

An insulating hollow cylindrical layer 46 is disposed about and joinedto member42. The layer 46 comprises a material such, for example, asalumina, porcelain, mica and boron nitride. However, the insulatingmaterial may be plasma jet sprayed on the outer surfaces of the innercylindrical member 42.

A plurality of inner bridging ring members 48 are disposed about andjoined to the insulating layer 46. The ring members 48 are electricallyinsulated from each other by means of insulating washers 50 comprisingmaterials, such as mica, or those selling under the trade name of Laviteor Mycalex (the latter being a glass-mica reaction product).

A plurality, of N-type thermoelectric material washer members 52 and aplurality of .P-type thermoelectric material, preferably previouslypreformed under pressure, washer members 54 are alternately disposed'onand joined to the bridging metal ring members 48.

A plurality of outer bridging metal ring members 56 are disposed on, andjoined to, the thermoelectric washer members 52 and. 54, the ringmembers 56 each contacting a pair of adjacent P and N-typethermoelectric washer members 52 and 54. The ring members 56 areelectrically insulated from each other by means of insulating washers58. The insulating washers 58 may comprise the same material as theinsulating washer 50. The washers 58 have a larger outside diameter anda larger inside diameter than the corresponding diameters of washers 50.

A hollow concentric insulating cylindrical layer 60 comprising amaterial such as that employed for layer'46, is disposed about, andjoined to, the outer ring members 56. The outer cylindrical metal member44 is disposed about and joined to the insulating cylindrical layer 60.

The components of element 30, such as the thermoelectric washer members52 and 54, bridging members 48 and 56, and the insulating washers 50 and58 are slipped into the space between cylindrical members 40 and 42 withtheir applied in sulating layers 46 and 60 already applied. The smallestpossible clearance is provided. The ends of the assembly are capped bywelding a disk at each end and the interior is evacuatedthrough a holeor tube left in place, which opening is then sealed off. The assembly isthen heated and compressed in an autoclave at pressures of up to 50,000p.s.i. to consolidate the whole into a bonded element.

The isostatic hot pressing operation provides an intimate and effectivemetallurgical bond between the bridging metal ring members 48 and 56 andthe adjoining thermoelectric washers 52 and 54 so as to provide a goodelectrical contact to each thermoelectric washer member 52 and 54whereby the thermoelectric washer members 52 and 54 are electricallyconnected in series. A good thermal conductivity relationship allowinggood heat flow is formed between the outer cylindrical metal member 44and the insulating cylindrical layer 60 and between the insulating layer60 and the metal ring members 56. Similarly, a good bond is formedbetween the inner hollow cylindrical member 42 and the inner cylindricalinsulating layer 46 and between the insulating layer 46 and the innermetal ring members 48. Member 42 is in compressive stress.

After removal of the welded ends, disks, electrical connector clamps 62and 64 may be then attached to form the thermoelectric device 30. Thedevice 30 may then be connected to'a load 66 by .means of electricalleads 68 and 70 attached to terminals 63 and 65 of clamps 62 and 64.

The inner cylindrical metal member 42 of the device 30 is particularlysuited to serve for passing high temperature gases and liquids so as tomake this the hot side, and the outer cylindrical metal member of thedevice can be exposed to the cooling medium to serve as the cold side ofa thermocouple. The inner cylindrical member may be conveniently heatedby passing hot water, steam, a flame or the like therethrough. The outercylindrical member may be cooled by flowing water or cold gases or airthereover. The difference in temperature between the hot side and coldsidewall cause an electrical current to be generated in thethermoelectric device by the phenomenon which is known in the art as theSeebeck effect. However, it should be understood that the inner metalmember may serve as the cold side and the outer metal member as the hotside.

Furthermore, the Peltier efiect may be employed on the device 30 toproduce refrigeration or cooling devices by passing electrical currentthrough the leads 68 and 70.

The preferred embodiment of this device 30 anticipates that in use themember 42 is the high temperature side. Thick wall inner metal member 42is in compression as a result of processing of the element 40 inautoclave. The thickness of the member 42 is selected so as to enablethe member 42 to withstand the temperature and pressure of the autoclavewithout failure or collapse. The hot pressing of the components of theelement 40 at the high temperature and pressure of the autoclave leavesthe metal of the thick wall inner member 42 in-a state of highcompression.

Therefore, when the member 42 is the hot side of the thermoelectricelement 40, the elevated temperature of the high temperature mediapassing through the hollow causes the member 42 to expand therebyenabling the member 42 to endeavor to remain in a good thermalconductivity relationship with adjoining components of the element 30.

With reference to FIG. 4 there is shown a thermoelectric element 80utilizing an alternate embodiment of the teachings of this invention.

The element 80 comprises a thin wall hollow inner cylindrical metalmember 32 and a concentric outer cylindrical metal member 84. Disposedwithin the hollow inner cylindrical metal member 82 is a suitable solidfuel source 86.

An insulating hollow cylindrical member 88 is disposed about and joinedto the member 82. The member 88 comprises a material such, for example,as alumina, porcelain, mica and boron nitride. However, the insulatingmaterial may be plasma jet sprayed on the outer surface of the innercylindrical member 82.

A plurality of inner bridging ring members 90 are disposed about andjoined to the insulating member 88. The ring members 90 are electricallyinsulated from each other by means of insulating washers 92 and 94comprising materials such, for example, as mica, or those selling underthe trade name of Lavite or Mycalex are alternately disposed between thering members 90. The only difference between the washers 92 and 94 isthe inside and outside diameter measurements.

A plurality of a first N-type thermoelectric material washer members 96and a plurality of first P-type thermoelectric material washer members98 are alternately disposed on and joined to the bridging metal ringmember 90.

A plurality of second N-type thermoelectric material washer members 100are disposed on, and joined to, the first N-type washer members 96. Aplurality of second P-type thermoelectric material washer members 102are disposed on, and joined to, the first P-type washer member 98. Thethermoelectric materials selected for comprising the members 96, 98, 100and 102 are chosen to provide the most efficient source ofthermoelectricity for the expected temperature gradient across eachmember 96, 98, 100 and 102 to be experienced during operation of theelement 80,

The employment of the plurality of two different N-type thermoelectricmaterials and the plurality of two different P- type thermoelectricmaterials is a technique variously known as thermal cascading" orsegmenting." As a specific example of this technique to illustrate thisparticular embodiment of this invention, the thermoelectric materialsmay be as follows:

Washer members 96-an N-type lead telluride alloy embodying a smallamount of lead iodide as an additive and commercially available underthe trade mark TEC-S-BN."

Washer members an N-type lead telluride alloy embodying a small amountof lead iodide as an additive and commercially available under the trademark TEGS-ZN."

Washer members 98-a P-type lead telluride-tin telluride alloycommercially available under the trade mark TEGS3 P."

Washer members 102a P-type lead telluride alloy commercially availableunder the trade mark TEGS2P."

Electrically insulating the members 100 from the members 102 from eachother is a plurality of insulating washers 104 and 106, alternatelydisposed on, and joined to, the corresponding insulation washers 92 and94. The washers 104 and 106 comprise the same materials as the washers92 and 94.

A plurality of outer bridging metal ring members 108 are disposed on,and joined to, the thermoelectric washer members 100 and 102, the ringmembers 108 each contacting a pair of composite P-type thermoelectricwasher members 96-100 and P-type thermoelectric washer members 98402.The ring members 108 are electrically insulated from each other means ofthe insulating washer 106.

A hollow concentric insulating cylindrical member 110 comprising amaterial such as that employed for reference numerals 88 is disposedabout and joined to the outer ring members 108. The outer cylindricalmetal member 84 is disposed about and joined to the insulatingcylindrical member 1 10.

This alternate embodiment of the teachings of this invention alsoenables the thin wall of the inner member 82 to be collapsed under theinfluence of the temperature and the pressure of the autoclave onto thesurface of the solid fuel source 86. The resulting structure thereforeenables a good thermal conductivity relationship to be retained betweenall components of the element 80 during operation of the element 80.

Although hot isostatic pressing alone will produce a satisfactorythermoelectric device in accordance with the teachings of thisinvention, it is desirable in many instances to interpose a cold workingor a cold compaction processing step between the assembly of thecomponents and the hot isostatic pressing of the components of thedevice. The purpose of this cold working or compaction process step isto remove substantially all of the assembly clearances and voids fromthe internal circuit structure of the element prior to exposing thecomponents of the element to the elevated temperatures of the hotisostatic pressing process. This precautionary cold working orcompaction step effectively limits the possibility of sublimation of anyof the component materials of the element, particularly thethermoelectric materials thereby preventing the resulting transport ofthe resultant vapors of the sublimed material through clearances andvoids in the internal circuit structure and subsequent condensation ofthe sublimed material vapors at undesirable locations within thestructure of the element.

Swagging, rocking-roll tube reducing and isostatic cold compressing aresuitable processes for effecting this intermediate process step on thethermoelectric elements. Usually, a specific thermoelectric element isdesigned mechanically to be compatible with a specific cold working or acold compaction process. Swagging and rocking-roll tube reducing willyield uniform overall diametral changes at the expense of a slightdistortion and/or elongation of the element in an axial direction.Isostatic cold compressing, performed in a liquid or a gas autoclave atabout room temperature and applied pressures of from 20,000 p.s.i. to100,000 p.s.i., will yield uniform diametral changes proportional to theelastic/plastic properties of the material or materials at any givencross-sectional plane along the axis of the element with substantiallyno attendant axial displacement. The temperatures of this cold Thereduction in the annular area between the inner and the inner tubesurfaces of the assembly. The assemblies were initially under a 10,000p.s.i. pressure at room temperature, and then heated to a temperature of650 C. while maintaining the same pressure and they were held at thattemperature and 2 Cold isostatic compressing.

this invention:

Example I Six thermoelectric elements similar to that shown in FIG. 3were assembled and fabricated by employing a prior art method asfollows:

The inner cylindrical member employed was a stainless steel hollow tubehaving a 0.376 in. 1D. and 0.426 in. OD. and having a 0.015 in. thicktube of boron nitride disposed thereon. The bridging contact ringmembers consisted of low carbon. steel and were of two different sizes.The inner contacts measured 0.460 in. [.D. and 0.514 in. OD. The outercontacts measured 0.76 in. ID. and 0.79 in O.D. The thermoelectricwashers employed consisted of P and N-type lead telluride. having adensity of 90 percent of theoretical and measuring 0.764 in. O.D. and0.516 in. [.D. The insulating material between alternate thermoelectricwashers and bridging contacts consisted of mica washers and were of twosizes. The inner insulating washers measured 0.764 in. O.D. and 0.460in. ID. The outer insulating washers measured 0.788 in. O.D. and 0.516in. ID. The outer bridging contacts had a 0.015 in. thick boron nitridetube disposed thereon. The outer cylindrical member consisted of astainless steel tube 10 in. long and measuring 0.870 in. O.D. and 0.821in. ID. The total gap space of the assembly in the radial direction was0.006 in.

Annular end plugs consisting of sheet stainless steel were then insertedand welded at the ends of the assembly to the inner and outer tubes andthe annular area between the inner and outer tubes of the assembly wasevacuated through a tube which was then sealed off. The assemblies wereeach disposed in a separate autoclave and treated in a similar manner.However, autoclave pressure was imposed on both the outer and outercylindrical members which results from any of the 5 pressure for 2hours. The assemblies were then cooled and the above-mentionedintermediate cold working process steps is pressure decreased to about4000 or 5000 p.s.i., the decrease generally less than percent. Thepercentage reduction of in pressure being linear with the decrease intemperature. The the annular area produced by this cold compacting stepgas pressure maintained in the autoclave was through the use renders asmall reduction during the hot isostatic pressing of OfhBhUm g theannular area fully adequate to secure metallurgical bond- The deVlCeS et st d by ln rllng a r h a er nd thering, so thatthe larger reductionsin the 1 percent to percent mocouples each bore. A temperature differene of 1 8 C. range are unnecessary Thus, 1 Percent m 6 Del-cam hot wasmaintained between the outer and inner cylindrical mem- Static reductionin b f ll adequate, and in some cases even bers when the outer side wascooled with water. The elements less than 1 percent is sufficient. l 5were evaluated for non'nalized thermal conductance and nor- Referringnow to Table I thereisshown a compilation of [hauled overall emcwncywhen Operate}! over 3 Period of data obtained from the fabrication ofeach of six different tlme- The data Pbtamed these tests 15 PlottedFIGS- 5 types of tubular thermoelectric elements in accordance with and6 and ldentlfied as Prlor elements- I the teachings of this invention.Particularly, the data of Table I Example 11 illustrates the magnitudesof the reduct1on 1n the annular cross-sectional area of each elementachieved as a result of an Three thin wall thermoelectric elementshereinafter intermediate cold working process step, as well as theaddiidentified as No. l, 2 and 3, were prepared and assembled. tionalreduction in the annular cross-sectional area achieved The basicconstruction of each element was the same as the as a result of hotisostatic pressing. The cold isostatic pressing l m nt h wn in FIG. 3except that the inner member of each and the hot isostatic pressingapplied pressure only to the ex- 0f the elements was f h hm W ll h ll wtube type and a terior surfaces of the elements. simulated solid fuelsource was employed. Each of the ele- TABLE I 52335 Cold processing Ilotisostatic pressing IEJEHEuJII W 4 Itcductibn O.D. LD. O.D. in annulur'lump. Pressure 0.1). in annular Element type (in.) (in.) Method (in.)area 1 C.) (p.s.i.) (in.) area 1 (Example I) 0. 870 0.375 Swaging 75010, 000 0. 840 8.3 2 (Example II). 1.625 0.500 do 750 10,000 1.570 7.3 3(Example IV). 1.700 0 500 Gold compactl 650 20, 000 1.660 5. 1 4 1. 700650 15, 000 1. 663 4. 7 5..- 2. 714 650 20, 000 2. 632 7. 1 6 1. 685 65020, 000 1. 642 6. 3

1 Percent of initial area.

ments was exactly the same as the prior art elements fabricated andtested in Example I except that elements 1, 2 and 3 were fabricated inaccordance with the teachings of this invention.

The inner cylindrical member employed was a stainless steel hollow tubehaving a 0.376 in. ID. and 0.426 in. O.D. and having a 0.015 in. thicktube of boron nitride disposed thereon. The bridging contact ringmembers consisted of low carbon iron and were of two different sizes.The inner contacts measured 0.460 in. LB. and 0.514 in. O.D. The outercontacts measured 0.760 in. ID. and 0.790 in O.D. The thermoelectricwashers employed consisted of P- and N-type lead telluride having adensity percent of theoretical and measuring 0.516 in. ID. and 0.764 in.O.D. The insulating material between alternate thermoelectric washersand bridging contacts consisted of mica washers and were of two sizes.The inner insulating washers measured 0.460 in. ID. and 0.764 in. O.D.The outer insulating washers measured 0.516 in. ID. and

0.788 in. O.D. The outer bridging contacts had a 0.015 in.

thick boron nitride tube disposed thereon. The outer cylindrical memberconsisted of three sections of stainless steel tube totaling 10 in. inlength and measuring 0.821 in. ID. and 0.870 in. O.D. The total gapspace of the assembly in the radial d rsst 2.919% i The outer bridgingcontact at each end contained a flange into which four steel conductorpins were inserted. These pins passed through tubular blocks ofelectrically insulating material comprising aluminum magnesium silicateand a stainless steel retaining ring. The pins were insulate from theretaining ring with small tubular washers of boron nitride. At each endof the devices a stainless steel end ring containing an evacuation tubewas welded to the inner and outer cylindrical member s The jointsbetween the three sections of the outer cylindrical member were weldedtogether such that joints were also formed between the outer cylindricalmember and the retaining rings. The annular region between the inner andouter cylindrical members was then evacuated through the evacuationtubes and the tubes were then sealed by welding.

Rod-type electrical resistance heaters measuring 0.375 in. OD. and 3.25in. long were inserted in the bores of the assembled elements such thatthe heater length was adjacent to the active thermoelectric circuit. Theremainder of the bore was filled with solid plugs of insulating materialcomprising aluminum magnesium silicate. A stainless steel solid plug wasthen welded into each end of the bore of the inner cylindrical member.

The elements so sealed were then passed through the dies of a swagingmachine, reducing the OD. of the element from 0.870 to 0.860 in. Thisrepresents a reduction in annular cross-sectional area of about 2.8percent, and effectively placed all of the layers in physical contactwith one another.

The elements were then disposed in an autoclave. The autoclave washeated with an internal electric furnace and pressurized by employinghelium gas. A pressure of7500 p.s.i. was imposed on the elements at roomtemperature. They were then simultaneously heated and additionallypressurized until a temperature of 650 C. and a pressure 10,000 p.s.i.were reached. These conditions of temperature and pressure weremaintained for 2 hours, after which the temperature and pressure weresimultaneously reduced until room temperature and a pressure of about5000 p.s.i. were reached. The remaining pressure was then removed andthe elements were taken out of the autoclave. The CD. of the elementshad been reduced to 0.840 inch, which represents an annular areareduction of about 5.7 percent from the preautoclave condition and atotal annular area reduction of about 8.3 percent from the preswagingcondition.

The ends of the devices were machined to expose the conductor pins ofthe thermoelectric circuit and the terminals of the rod heater. Testingwas accomplished using the rod heater and an external blower. Atemperature difference of 168 C.

was maintained across the radius of each device by heating the innercylindrical member with the rod heater and cooling the outer cylindricalmember with the forced air form the blower.

The elements were evaluated for normalized thermal conductance andnormalized overall efficiency when operated over a period of time. Thedata obtained are graphed as shown in FIGS. and 6. The superiority inthe performance of elements 1, 2 and 3 is clearly evident and can beattributed solely to the novel fabrication process of this invention.

Example 111 Two thick wall thermoelectric elements, hereinafteridentified as elements No. 4 and 5, were assembled. The basicconstruction of each element was similar to that shown in FIG. 3.

The inner cylindrical member was a Hastelloy B hollow tube having a0.500 in. ID. and 0.650 in. OD. and having a 0.018 in. thick tube ofboron nitride disposed thereon. The bridging contact ring membersconsisted of low carbon and and were of two different sizes. The innercontacts measured 0.690 in. [.D. and 0.791 in. OD. The outer contactsmeasured 1.291 in. ID. and 1.330 in. OD. The thermoelectric washersemployed consisted of P- and N-type lead telluride having a density 93percent of theoretical and measuring 0.792 in. ID. and 1.290 in. 0.1).The insulating material between alternate thermoelectric washers andbridging contacts consisted of mica washers and were of two sizes. Theinner insulating washers measured 0.690 in. ID. and 1.289 in. OD. Theouter insulating washers measured 0.793 in. ID. and 1.330-in. OD. Theouter bridging contacts had a 0.020 in. thick boron nitride tubedisposed thereon. The outer cylindrical member consisted of threesections of stainless steel tube totaling 14.4 in. in length andmeasuring 1.374 in. ID. and 1.625 in. OD. The total gap space of theassembly in the radial direction was 0.006 in.

The outer bridging contact at each end contains a flange into which foursteel conductor pins are inserted. These pins pass through tubularblocks of insulating material comprising magnesium aluminum silicate anda stainless steel retaining member. The retaining member was welded tothe inner cylindrical member. The pins were insulated from the retainingmember with small tubular washers of boron nitride. At each end of theelements a stainless steel end ring containing an evacuation tube waswelded to the inner and outer cylindrical members. The joints betweenthe three sections of the outer cylindrical member were welded togethersuch that joints were also formed between the outer cylindrical memberand the retaining members. The annular region between the inner andouter cylindrical members was then evacuated through the evacuationtubes and the tubes were then sealed by welding.

The elements were then passed through the dies of a swaging machine,reducing the OD. from 1.625 to 1.590 in. This represents a reduction inannular cross-sectional area of about 4.7 percent and effectivelyeliminated internal clearances and placed all the layers in closecontact with one another. A stainless steel of solidplug was then weldedinto each end of the bore of the inner cylindrical member of eachdevice.

The devices were then disposed in an autoclave which could bepressurized with helium gas and heated with internal coils. A pressureof 7500 p.s.i. was imposed on the elements at room temperature. Theywere then simultaneously heated and additionally pressurized until 750C. and 10,000 p.s.i. were reached. These conditions were maintained for2 hours, after which the temperature and pressure were simultaneouslyreduced until room temperature and about 7500 p.s.i. were reached. Thenthe remaining pressure was removed and the elements were taken out ofthe autoclave. The outside diameter of the elements had been reduced to1.570 inch, which represents an area reduction of about 2.8 percent fromthe preautoclaving condition and a total area reduction of about 7.3percent from the preswaging condition.

The ends of the elements'were machined to expose the conductor pins ofthe thermoelectric circuit and to remove the plugs from the bores of theinner cylindrical members.

Testing was accomplished by inserting a rod heater in the bore to heatthe inner surface and using forced air from a blower to cool the outersurface. A temperature difference of about 450 C. was maintained acrossthe radius of the devices in this manner.

The elements were evaluated for normalized thermal conductance andnormalized overall efficiency when operated over a period of time. Thedata obtained are plotted on a graph as shown in FIGS. 5 and 6, aselements No. 4 and 5.

Example IV A thick wall thermoelectric element identical to thoseelements fabricated and described in Example 111, except for having aninner cylindrical member made of Inconel X-750 and an outer cylindricalmember measuring 1.700 in. OD, was assembled, evacuated, and the endshermetically sealed by welding. This element is described hereinafter aselement No. 6.

Along, close-fitting solid stainless steel center rod and two shortsolid stainless steel end plugs, all closely fitting the bore wereinserted as a solid column into the bore of the inner cylindrical memberand the end plugs were seal welded to the ends of the member. Theelement was then cold isostatic compressed in a liquid autoclave at roomtemperature, with an iso static pressure of 50,000 p.s.i. being appliedto the exterior surfaces of the element only. This cold isostaticcompression process reduced the outer diameter from 1.700 to 1.675inches, which represented a reduction in annular cross-sectional area ofabout 3.2 percent, and effectively placed all the layers of the elementin an intimate physical contact with one another.

The thermoelectric element was then disposed within an autoclave whichcould be pressurized with helium gas. The autoclave was heatedinternally with electrical coils. While at room temperature helium at apressure of 15,000 p.s.i. was imposed on the exterior surfaces of theelement only. The element was then simultaneously heated and thepressurized increased until the conditions of temperature of 650 C. anda pressure of 20,000 p.s.i. were reached. The elevated temperature andhigher pressure were maintained on the element for 2 hours, at whichtime the temperature and the pressure were simultaneously reduced untilthe helium was at room temperature and a pressure of 15,000 p.s.i. wasachieved. The helium gas was released from the autoclave and theprocessed element removed from the autoclave.

The outside diameter of the element had been reduced to 1.660 incheswhich represented an annular area reduction of about 2.0 percent fromthe prehot isostatically compressed condition, there being a totalannular area reduction of about 5.1 percent from the precold compactioncondition;

The element was prepared for testing, tested and the results evaluatedin a similar manner as the elements of Example III. The data obtainedfrom the tests is plotted in FIGS. 5 and 6 and denoted as element No. 6.

A review of the data plotted in FIG. 5 reveals that the elementutilizing and fabricated by means of the teachings of this invention arefar superior to those elements manufactured by the prior art method.

It is to be noted that the elements embodying the teachings of thisinvention still have a normalized thermal conductance of 0.95 of theoriginal values or better after 900 hours of operating time. In theelements embodying the teachings of the prior art the normalized thermalconductancedeteriorated to below 0.7 after less than 600 hours ofoperating time.

In reviewing the collected data plotted in FIG. 6, it will be seen thatthe elements embodying the teachings of this invention have a highernormalized efficiency for a greater operating time than the elementsembodying the teachings of the prior art. After 600 hours of operatingtime the elements embodying the teachings of this invention dropped to anormalized overall efficiency of approximately 0.9 of the initial valuesand remained at this efficiency for more than 300 hours more. Bycomparison the prior art processed elements decreased to an efficiencyof from approximately 0.6 to 0.85 in 500 hours of operating time orless.

While the invention has been described with reference to particularembodiments and examples, it will be understood, of course, thatmodifications, substitutions and the like may be made therein withoutdeparting from its scope.

We claim as our invention:

1. In a process for producing an integral thermoelectric elementcomprising a hollow inner cylindrical member, a concentric outercylindrical member and a body of thermoelectric material disposed in thecompartment defined by the outer surface of the inner member and theinner surface of the outer member, the steps comprising:

1. disposing the body of thermoelectric material in the compartmentbetween the inner and the outer member,

2. sealing the ends of the compartment and the hollow of the innermember so that no fluid may enter therein,

3. evacuating the sealed body of thermoelectric material in thecompartment,

4. compressing the members and the thermoelectric material at atemperature from about 250 C. to slightly below the melting temperatureof the lowest melting material comprising the element by applying anisostatic pressure of from 5,000 p.s.i. to 50,000 p.s.i. only toexterior surfaces of the element until the element has been plasticallydeformed at its exterior surfaces and reduced in crosssectional area bya value of from about 1 percent to percent to provide at least anintimate physical contact between the body of thermoelectric materialand the inner and outer members, and the inner cylindrical -member is incompression after the isostatic pressure is removed.

2..The process of claim 1 in which compressing the members results in ametallurgical bond between the body of thermoelectric material and theinner and outer members of the element.

3. The process of claim 1 in which the hollow inner cylindrical memberhas a thin wall, sealing and evacuating a solid heat source disposedwithin the hollow of the inner member, and collapsing the thin wall uponthesolid heat source by the application of the isostatic pressure at theelevated temperature.

4. The process of claim Lincluding cold'working the componentscomprising the element after assembly to reduce internal clearances andclose gross voids prior to being subjected to the isostatic pressure atan elevated temperature.

5. The process of claim 4 in which the cold working is performed byisostatic pressure applied only to the exterior surface of the assembledcomponents at room temperature.

6. The process of claim 1 in which the hollow of the inner member isfilled with an inert gas and sealed prior to the assembled element beingsubjected to the isostatic pressure at an elevated temperature. V

7. In a process for producing a thermoelectric element, the stepscomprising: I

. disposing a plurality of inner bridging metal ring members on aninsulated hollow cylindrical metal member, the ring members beingelectrically insulated from each other,

2. disposing a plurality of washers of thermoelectric material on thering members,

3. disposing a plurality of outer bridging metal ring members on saidthermoelectric washers, the ring members being electrically insulatedfrom each other, disposing an insulated outer cylindrical metal memberon the ring members, the assembly having a gap space of not above about1 percent of the diameter of the outer cylindrical member,

4. sealing the ends of the cylindrical metal members to provide a sealedenclosure for the components therein,

5. sealing the end of the hollow cylindrical metal member so that nofluid can enter therein, 7

6. evacuating the internal spaces within the sealed assembly,

and

7. compressing the members and the components contained therebetween ata temperature of from 250 C. to slightly below the melting temperatureof the lowest melting temperature component in the assembly by applyingan isostatic pressure of from 5,000 p.s.i. to 50,000 p.s.i. to theexterior surface only of the assembly until plastic deformation occursat the outer portions and until there is a reduction in cross-sectionalarea of the space between the inner and outer cylindrical members offrom 1 percent to 15 percent whereby to provide a metallurgical bondbetween the bridging metal ring members and the thermoelectric washersso that an applied or induced electric current will meet alow electricalresistance flow between said washers, and the element in use at elevatedtemperatures maintains a high electrical and thermal conductivi- 8. "lhe process of claim 7 including:

cold working the assembled members and components of the element priorto compressing the members and the components isostatically at anelevated temperature to reduce internal clearances within the element.

9. The process of claim 8 in which cold working is performed byisostatic pressure on exterior surfaces only of the element at aboutroom temperature.

10. The process of claim 7 in which the hollow inner cylindrical memberhas a thin wall, sealing and disposing a solid heat source disposedwithin the hollow of the inner member, and collapsing the thin wall uponthe solid heat source by the application of the isostatic pressure atthe elevated temperature.

11. The process of claim 7 in which the hollow of the inner portion ofthe washer.

13. The process of claim 9 in which the washers of thermoelectriccomprise at least two varieties for each type of thermoelectric materialeach variety having of same type thermoelectric material having anoptimum range of thermoelectric efficiency different from each of theother varieties, one variety of one type thermoelectric materialcomprising one radial portion of the washer and a second variety of saidone type thermoelectric material comprising the second radial portion ofthe washer.

1. In a process for producing an integral thermoelectric elementcomprising a hollow inner cylindrical member, a concentric outercylindrical member and a body of thermoelectric material disposed in thecompartment defined by the outer surface of the inner member and theinner surface of the outer member, the steps comprising:
 1. disposingthe body of thermoelectric material in the compartment between the innerand the outer member,
 2. sealing the ends of the compartment and thehollow of the inner member so that no fluid may enter therein, 3.evacuating the sealed body of thermoelectric material in thecompartment,
 4. compressing the members and the thermoelectric materialat a temperature from about 250* C. to slightly below the meltingtemperature of the lowest melting material comprising the element byapplying an isostatic pressure of from 5,000 p.s.i. to 50,000 p.s.i.only to exterior surfaces of the element until the element has beenplastically deformed at its exterior surfaces and reduced incross-sectional area by a value of from about 1 percent to 15 percent toprovide at least an intimate physical contact between the body ofthermoelectric material and the inner and outer members, and the innercylindrical member is in compression after the isostatic pressure isremoved.
 2. sealing the ends of the compartment and the hollow of theinner member so that no fluid may enter therein,
 2. The process of claim1 in which compressing the members results in a metallurgical bondbetween the body of thermoelectric material and the inner and outermembers of the element.
 2. disposing a plurality of washers ofthermoelectric material on The ring members,
 3. The process of claim 1in which the hollow inner cylindrical member has a thin wall, sealingand evacuating a solid heat source disposed within the hollow of theinner member, and collapsing the thin wall upon the solid heat source bythe application of the isostatic pressure at the elevated temperature.3. evacuating the sealed body of thermoelectric material in thecompartment,
 3. disposing a plurality of outer bridging metal ringmembers on said thermoelectric washers, the ring members beingelectrically insulated from each other, disposing an insulated outercylindrical metal member on the ring members, the assembly having a gapspace of not above about 1 percent of the diameter of the outercylindrical member,
 4. The process of claim 1 including cold working thecomponents comprising the element after assembly to reduce internalclearances and close gross voids prior to being subjected to theisostatic pressure at an elevated temperature.
 4. compressing themembers and the thermoelectric material at a temperature from about 250*C. to slightly below the melting temperature of the lowest meltingmaterial comprising the element by applying an isostatic pressure offrom 5,000 p.s.i. to 50,000 p.s.i. only to exterior surfaces of theelement until the element has been plastically deformed at its exteriorsurfaces and reduced in cross-sectional area by a value of from about 1percent to 15 percent to provide at least an intimate physical contactbetween the body of thermoelectric material and the inner and outermembers, and the inner cylindrical member is in compression after theisostatic pressure is removed.
 4. sealing the ends of the cylindricalmetal members to provide a sealed enclosure for the components therein,5. sealing the end of the hollow cylindrical metal member so that nofluid can enter therein,
 5. The process of claim 4 in which the coldworking is performed by isostatic pressure applied only to the exteriorsurface of the assembled components at room temperature.
 6. The processof claim 1 in which the hollow of the inner member is filled with aninert gas and sealed prior to the assembled element being subjected tothe isostatic pressure at an elevated temperature.
 6. evacuating theinternal spaces within the sealed assembly, and
 7. In a process forproducing a thermoelectric element, the steps comprising:
 7. compressingthe members and the components contained therebetween at a temperatureof from 250* C. to slightly below the melting temperature of the lowestmelting temperature component in the assembly by applying an isostaticpressure of from 5,000 p.s.i. to 50,000 p.s.i. to the exterior surfaceonly of the assembly until plastic deformation occurs at the outerportions and until there is a reduction in cross-sectional area of thespace between the inner and outer cylindrical members of from 1 percentto 15 percent whereby to provide a metallurgical bond between thebridging metal ring members and the thermoelectric washers so that anapplied or induced electric current will meet a low electricalresistance flow between said washers, and the element in use at elevatedtemperatures maintains a high electrical and thermal conductivity. 8.The process of claim 7 including: cold working the assembled members andcomponents of the element prior to compressing the members and thecomponents isostatically at an elevated temperature to reduce internalclearances within the element.
 9. The process of claim 8 in which coldworking is performed by isostatic pressure on exterior surfaces only ofthe element at about room temperature.
 10. The process of claim 7 inwhich the hollow inner cylindrical member has a thin wall, sealing anddisposing a solid heat source disposed within the hollow of the innermember, and collapsing the thin wall upon the solid heat source by theapplication of the isostatic pressure at the elevated temperature. 11.The process of claim 7 in which the hollow of the inner member is filledwith an inert gas and sealed prior to the assembled element beingsubjected to the isostatic pressure at an elevated temperature.
 12. Theprocess of claim 7 in which each of the washers of thermoelectricmaterial comprise at least two different varieties of the same typethermoelectric material each variety of thermoelectric material havingan optimum range of thermoelectric efficiency different from each of theother varieties, one variety of one type thermoelectric materialcomprising one radial portion of the washer and second variety said onetype thermoelectric material comprising the second radial portion of thewasher.
 13. The process of claim 9 in which the washers ofthermoelectric comprise at least two varieties for each type ofthermoelectric material each variety having of same type thermoelectricmaterial having an optimum range of thermoelectric efficiency differentfrom each of the other varieties, one variety of one type thermoelectricmaterial comprising one radial portion of the washer and a secondvariety of said one type thermoelectric material comprising the secondradial portion of the washer.